Welding electrodes are conventionally formed by multi-step drawing of relatively ductile material from rods. As the material hardens from cold working (dislocation generation) during each drawing step, iterative annealing is performed to erase such cold working, reduce the strength and improve the ductility of the material for subsequent drawing steps. Superalloy materials which are used to manufacture gas turbine engines have extremely high strength and low ductility, even at very high temperatures. Due to these characteristics, annealing has limited ability to improve the ductility of superalloy materials. As a result, it is difficult to form welding electrodes from some superalloy materials, particularly the gamma prime strengthened alloys with high aluminum and titanium content, such as alloys 247, 738, 939, etc.
The term “superalloy” is used herein as it is commonly used in the art; i.e., a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures. Superalloys typically include a high nickel or cobalt content. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g. IN 738, IN 792, IN 939), Rene alloys (e.g. Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g. CMSX-4) single crystal alloys.
Shielded metal arc welding (SMAW) is a manual arc welding process that uses a consumable stick electrode covered with a flux. An electric current is used to form an electric arc between the electrode and the workpiece, thereby melting the electrode and a portion of the workpiece to form the weld joint. Because of the simplicity and versatility of SMAW, it is one of the world's most popular welding processes. One limitation of SMAW is the use of a rigid stick electrode, which generally prevents its application to continuous or automatic welding processes which typically feed a bendable electrode from a spool.
Flux-cored arc welding (FCAW) uses a tubular alloy electrode containing flux. Because the flux can be in powder form and is captured within the tubular alloy sheath, the electrode can be bent and stored in coil form, thereby facilitating continuous and automatic welding.
Conventional electrodes for welding have been formed from ductile material, such as stainless steel. For example, United States Patent Application Publication No. US 2004/0173592 A1 discloses an electrode including a stainless steel sheath encapsulating a core containing alloying and flux materials. Conventional electrodes have also been developed for welding some of the lower strength superalloy materials. For example, United States Patent Application Publication No. US 2012/0223057 A1 discloses a coated electrode used for gas tungsten arc welding of certain superalloys. The electrode includes a solid core formed from the superalloy material and an outer coating of flux material.